Anisotropic Electrowetting on Wrinkled Surfaces: Enhanced Wetting

Subscriber access provided by READING UNIV

Article

Anisotropic electrowetting on wrinkled surfaces: Enhanced wetting and dependency on initial wetting state Vartika Parihar, Saumyadwip Bandyopadhyay, Soumen Das, and Sunando DasGupta Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03467 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Anisotropic electrowetting on wrinkled surfaces: Enhanced wetting and dependency on initial wetting state Vartika Parihar1, Saumyadwip Bandyopadhyay2, Soumen Das2,3, Sunando Dasgupta1,2* 1

Department of Chemical Engineering, Indian Institute of Technology Kharagpur, Kharagpur-

721302, West Bengal, India. 2

Advanced Technology Development Centre, Indian Institute of Technology Kharagpur,

Kharagpur- 721302, West Bengal, India. 3

School of Medical Science and Technology, Indian Institute of Technology Kharagpur,

Kharagpur- 721302, West Bengal, India.

_____________________________________ *Corresponding author, e-mail: [email protected]

ABSTRACT: Electrowetting on dielectric (EWOD) on uni-directional microstructured surfaces has recently evoked significant interest as they can modulate the effect of electrowetting, and can thus find applications in directional wetting in microfluidic systems. However, the dependency of such EW phenomenon on their initial state of wetting and anisotropy are far from being well understood. The current study addresses the initial wetting states and their implication on the anisotropic electrowetting using a wrinkled EWOD platform. Herein we demonstrate a facile stamp-less and mask-less structure generation technique to fabricate wrinkles of varying topography. Further, we have demonstrated alteration in the interfacial wetting conditions by modulating the wrinkle topography, and its effect on the droplet behavior during electrowetting. The capillary wicking assisted electrowetting on these wrinkled surfaces is in specific direction dictated by the ordered wrinkles and prompts enhanced spreading of the droplet. We also demonstrate that while the enhancement of uni-directional electrowetting is stronger in conformal wetting state surfaces; composite wetting state surfaces depict a reversal in anisotropy. 1

ACS Paragon Plus Environment

Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 30

KEYWORDS: EWOD, wrinkles, conformal wetting, composite wetting, anisotropic wetting, microfluidics

1. INTRODUCTION Tuning of surface wettability using passive means, i.e. by surface texturing1–3and by active means i.e. by external stimuli such as electrical forces4, optical forces5, magnetic forces6, thermocapillary forces7, and surface acoustic waves8 etc., has evoked significant interest due

to

its

applications

in

digital

microfluidics9,

lab-on-a-chip

devices10,

droplet

transportation11etc12–14. Among the above-mentioned external agents used to tune the wettability, electrical forces4 have drawn significant interest due to its several advantages4 like, rapid movement of the droplet, higher maneuverability, choice of liquid, higher change in wetting properties thus, making it more efficient in droplet actuation,15 dispensing,16 mixing,17 splitting18, “lab-on-a-chip” devices19, adjustable lenses,20 and display technology21.

Recently, several

research groups have simultaneously used both passive and active means of tuning surface wettability by fabricating micro-structured surfaces for EWOD22–26 applications. They have demonstrated that the simultaneous use of both the passive and active means, to be superior in achieving higher wettability changes due to the transition from the Cassie-Baxter state (airliquid-solid composite interface) to the Wenzel state (solid-liquid interface i.e. complete wetting). To fabricate these textured EWOD platforms the techniques they have used are optical lithography,27 3D diffuser lithography,28 soft lithography,29micromachining and etching process,30 deep reactive ion etching,31 which prove to be elaborate and require sophisticated micro-fabrication facilities. Furthermore, the techniques are limited by the feature size of the available mask or the stamp. The first approach towards mask-less and stamp-less topography was proposed by Whitesides and his research group32. They introduced a highly ordered micro-wrinkled surface by introducing rectangular ridges on PDMS by soft lithography followed by metal deposition. Lately, many strategies for the formation of micro33–35 or nano36wrinkles have been reported. Recently, Maji et al.37 have obtained a highly ordered micro wrinkled surface by nichrome deposition on a stretched, soft-lithographically patterned PDMS substrate. A plethora of applications of these strain induced wrinkled surfaces such as droplet movement,36 slippery 2


Page 3 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

surfaces,34 flexible sensors,37 cell adhesion and alignment38, have been investigated. The uniqueness of these wrinkled surface apart from their facile fabrication technique lies in the anisotropic wetting characteristics39–41. Taking cognizance of the facile technique of fabrication and the directional wetting we investigated the possibility of such wrinkled substrates as EWOD applications. To the best of the author's knowledge electrowetting on these wrinkled surfaces has rarely been studied. In the present work, highly ordered micro wrinkled surfaces are prepared for EWOD applications. Unlike the process described in Maji et al., the present approach does not require any stamp for the creation of micro wrinkled surface. However, once the wrinkles are formed on nichrome, they are transferred to the PDMS (on ITO coated glass) using a soft lithography technique. The microscale roughness (buckles) is created by applying unidirectional stretching of the PDMS film followed by deposition of a nichrome film on it through sputtering. The method relies on the preferential stress distribution in the elastomer, in one direction by incorporating high aspect ratio rectangular strips of PDMS, in place of the patterned PDMS which were obtained by complex soft lithography37. A series of ordered buckles with a specific amplitude and wavelength is generated upon relaxation. It has been observed that changing the applied mechanical strain prior to the sputtering process results in varying topography. The wrinkled surfaces are characterized on the basis of their feature size and subsequently by their wetting behavior. The influence of wrinkle feature size and initial wetting states on EWOD behavior are also explored. Additionally, careful scrutiny of the published literature reveals that most of the micro-structured surfaces have a drawback of lower change in the CAs upon actuating the electric field in EWOD25,26,42,43, and a large change in the contact angle during EWOD has rarely been reported44. Herein, we report higher CA variation during EWOD on hierarchically rough hydrophobic PDMS surfaces, produced by the sputtering process. Most importantly, an anisotropic spreading on these wrinkled surfaces has been observed during EWOD experiments. This approach is clearly distinct from an earlier work by Vrancken et al., where a surface with parallel rectangular strips was proposed to show reversible EWOD with less energy barrier along the strips45. However, it involved a lithographic process for fabrication and was limited by the need of different stamps for getting varying feature size. We also show that the proposed method is able to vary the feature size by varying only one parameter, namely 3


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 30

the applied pre-strain and can be utilized to manipulate the liquid in one direction with a lower energy barrier.

2. MATERIALS AND METHODS 2.1. Master Preparation. The PDMS samples were prepared by mixing 10 part of Sylgard 184 (Dow-Corning Corporation, USA) elastomer with 1 part of cross linker in (w/w) ratio. This was followed by degassing in vacuum for 20 minutes to remove the air bubbles formed during the mixing process of the elastomer and cross linker. The PDMS was poured on pre-cleaned rectangular glass slides (50mm×25mm), and spun in two steps: at 300 rpm for 20 seconds for even distribution, and later at 500 rpm for 30 seconds. The thin film, thus formed was cured at 95 °C for 4 hours in a hot air oven for complete polymerization of PDMS. The thickness of the PDMS film was measured by a surface profilometer (Dektak) and found to be 500±20 µm. After cooling, the PDMS was peeled off from the glass slide and cut into rectangular pieces (48 mm × 12 mm). These were then fixed to a stretching device (Fig. 1) wherein one end of the PDMS film was clamped firmly to its initial position while the other end, was stretched by varying amounts of pre-strain (denoted by ‘ χ ’) using a screw-nut mechanism, at a uniform rate. The films were subjected to pre-strains varying from 0 % to 80 % of the original size. The deposition of a nichrome layer on these stretched PDMS elastomers was carried out through sputtering process. The sputtering was carried out under optimized conditions (Sputtering power 50W, time 10min) in order to get a crack-free thin film of nichrome. Subsequently, the strain was gradually released and the sample was attached to a glass slide by oxygen plasma treatment. By varying the stretching amount, wrinkles of different wavelength and amplitude were obtained on the nichrome surface.

4


Page 5 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 1 Schematic of the mould preparation. 2.2. Transfer of the wrinkles. The topography of prepared wrinkled surface obtained by varying the pre-stretching condition was transferred to a soft elastomer by a micro molding process. Initially Sylgard 184 film was spin coated on an ITO glass slide by the following two steps: 1) at 500 rpm for 30 seconds and 2) 3000 rpm for 70 seconds and was then cured at 90°C. This procedure was carried out to obtain a uniform thickness (13µm) Sylgard 184 film. Again Sylgard 184 film of the same thickness (i.e. 13 µm) was spin-coated following the same protocol. The structured side of the buckled nichrome film was then placed over this PDMScoated glass slide and a constant pressure of 150Pa is applied. The whole arrangement was kept in a convection oven at 35°C for 24 hours for proper replication. The thickness of the first layer was carefully optimized to avoid electrolysis at the crests of the features (where the film would be the thinnest) during the electrowetting experiments. 2.3. Surface characterization. The substrates were explored using surface profilometer (VeecoDektak 150, USA) and Scanning Electron Microscopy (JEOL, JAPAN, JSM-7610F) 5


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 30

(shown in Fig. 2), and the trend lines are shown in Fig. 3. The wetting behavior was studied by measuring the contact angles of a DI water droplet of 2µl volume on these substrates, using a Goniometer (Rame Hart, USA), images along with the contact angle values are shown in Fig. 4. 2.4. Identification of interfacial wetting state. To investigate the wetting state, a fluorescent dye containing water droplet (10 µl) was dispensed on the wrinkled substrate and removed within 30 seconds. This time is significantly smaller as compared to the complete evaporation time i.e. 20 minutes. Thus, the effect of dye accumulation near the contact line during evaporation is not a predominant factor and is unlikely to alter the results of the droplet wetting state. The substrates were then studied using fluorescence microscopy on an inverted microscope (Olympus IX71) to examine the droplet footprint on the surface and the results are shown in Fig. 5. The area under the droplet that comes in close contact with the substrate is characterized by the bright fluorescent colour of dye under a microscope and indicates the mode of contact and hence the possible wetting state of the droplet on the wrinkled surfaces, as will be discussed later. 2.5. Electrowetting experiment. The EWOD experiments on these wrinkled surfaces were conducted under ambient conditions. The potential was applied across the droplet (a region of the ITO coated glass is scrapped off the PDMS layer for electrical connections), using a DC power source (Keithley, USA). We first examined the variations of the apparent contact angles (CA) of the droplet on various wrinkled surfaces under increasing potentials using a goniometer. To accurately measure the droplet anisotropy during electrowetting, the droplet footprint diameters (in the direction parallel and perpendicular to the wrinkle), were measured using a stereo microscope (Leica MZ 125). The potential was altered in increments of 100 V and allowed to reach equilibrium before further change in the potential.

3. RESULTS AND DISCUSSION 3.1. Surface Characterization. Upon relaxation of the stretched PDMS film after sputtering, wrinkles on the surface appear spontaneously due to generated stress as a result of the mismatch in both the coefficients of elasticity and the coefficients of thermal expansion of the PDMS and the thin nichrome layer. Microscopic examination of the different areas of the wrinkled substrate 6


Page 7 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

has confirmed that the buckling occurred only in two dimensions. The wrinkles are characterized by their amplitude ( A ) and wavelength ( λ ).It has been found that the wrinkle parameters vary with the applied pre-strain ( χ ).SEM images and surface profilometer data of the two substrates subjected to different χ , ( χ = 20% and χ = 80% ) are shown in Fig. 2, and the values of A and

λ are plotted against their corresponding χ in Fig. 3. SEM observation reveals that wrinkles formed in the relaxed film are parallel to each other and aligned perpendicular to the stretching direction. The uni-directional and ordered wrinkles imparts roughness to the surface. This roughness has a strong directional dependency and is maximum in the direction perpendicular to the wrinkles and is negligible in the direction parallel to them. Compactness of buckles enhances at higher pre-straining condition. The profilometer data are analyzed to verify that the results are consistent with the conservation of mass by comparing the areas of smooth and the wrinkled surfaces for different frequency and amplitude.

Figure 2(a) SEM images of the substrate for χ = 20% , (b) Surface Profilometer result for the substrate, χ = 20% , (c) SEM image of the substrate for χ = 80% , (d) Surface Profilometer result for the substrate, χ = 80% . 7


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 30

Figure 3 Variation of the amplitude and wavelength with the pre-strain. It is also clear from the results presented in Fig. 3 that A monotonically increases from about 3.55 µm to 13 µm for an increase in χ from 20% to 80%; whereas, λ decreases from 47 µm to 28 µm. The surface profilometer data for the substrates depict a sinusoidal nature of the wrinkles that are fitted using the following expression46  2π x  y = Asin    λ 

(1)

and the roughness factors ( r ) of the wrinkled surface for the two perpendicular directions are evaluated by46, 1

2 2   2π A  2π x    ∫0  1 +  λ cos  λ    dx  = 

λ

r⊥ =

Aract ,x Arprj ,x

λ

r =

(2)

Aract ,z =1 Arprj ,z

Where, Aract , x , Arprj , x , Aract , z and Arprj , z are the actual area and the projected area in X and Z directions respectively. The above observations indicate that higher χ results in compact buckles with higher amplitude (lower λ and higher A ) and the value of the roughness factors in the perpendicular direction increases from 1.054 for the substrate with χ = 20% to 2.574 for

χ = 80% (Table1), whereas the roughness in the parallel direction remains constant (equal to 1); 8


Page 9 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

signifying negligible roughness for all the substrates. Thus, an effect of pre-strain on the compactness of the wrinkles has been established quantitatively, as depicted in Fig. 3. 3.2 Anisotropic wetting on wrinkled substrates. Since the substrates have uni-directional ordered wrinkles, the contact angles may have directional dependence and hence two contact angles are measured for the droplets placed on the substrate, namely i) along the wrinkles hereby referred to as the ‘ θ ’ and ii) across the wrinkles hereby referred to as ‘ θ⊥ ’. The droplet was analyzed in two orthogonal directions separately using a goniometer to measure the angles. The experiments (with droplets of the same volume) are repeated at least five times and the average values of the angles in each direction are reported. Also, the variation in the surface topography obtained by varying the pre-strain will have a prominent effect on the wettability. To examine this further, we have measured contact angles ( θ ) on different substrates for the droplet, and the corresponding images of the droplet along with the value of contact angles are shown in Fig. 4. Also, we have measured the contact angle on flat PDMS and is found to be 113°.

Figure 4 Contact angles subtended by a DI water droplet of 2 µl volume on surfaces with prestrains of 20%, 35%, 50%, 65%, and 80% respectively; Figures (a-e) show contact angles measured along the direction of wrinkles, θ ; whereas, (f-j) show θ⊥ on the same surfaces respectively. It can be observed from Fig. 4, that θ⊥ for all the substrates are significantly higher than the corresponding θ values. It is likely that upon dispensing a droplet on these wrinkled surfaces, the wrinkles under the droplet act as micro-capillaries and cause the droplet to 9


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 30

spread in a specific direction (parallel to the wrinkles) more as compared to the other. Also, θ⊥ increases rapidly with χ , e.g., from 122° for χ = 20% to 150° for χ = 80% whereas θ varies moderately from 113° for χ = 20% to 119° for χ = 80% . It is due to the increase in the roughness factor in the perpendicular direction, whereas, the roughness factor remains constant in the parallel direction as mentioned in Section 3.1.These differences between the values of

θ depicts anisotropic wetting and deformation of the droplet on the wrinkled substrates and are quantified by the eccentricity ( ε ) of the droplet which is defined as the ratio of the diameters at the base ( D / D⊥ ) and is shown in Fig. 5 along with the corresponding value of χ .This wrinkle engendered anisotropy of the substrate, increases with the compactness of the wrinkles due to higher capillary suction. Also, the higher amplitude of the wrinkles poses a greater energy barrier for the droplet, to spread perpendicular to the wrinkles.

Figure 5 The top view of 2 µl DI water droplets , on surfaces with increasing order of pre-strain, (a) χ = 20% , (b) χ = 35% , (c) χ = 50% , (d) χ = 65% , and (e) χ = 80% , respectively. It can be observed from Fig. 5 that the anisotropy initially increases with an increase in χ , but only up to χ = 50% , (from ε = 1.1 for χ = 20% to ε = 1.18 for χ = 50% ).The eccentricity decreases on further increase in the values of pre-strain ( ε = 1.05

for

χ = 65% and χ = 80% ). The results indicate that the capillary effect provided by the unidirectional wrinkles is stronger for lower values of χ (i.e. up to 50%) as compared to the substrates subjected to higher pre-strain. As the wrinkles become more compact with increase in the values of pre-strain (higher amplitude and lesser wavelength), it is likely that at higher prestrain the droplet is not making a conformal contact with the surface and thus not experiencing 10


Page 11 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

the strong wicking action prevalent at lower pre-strains. This underscores the need for further examination of the interfacial wetting states of the droplet on different substrates. 3.3. Identification of interfacial wetting state. It is likely that the droplet will be in either of the two possible states - it either sits conformal to the wrinkles, wetting the complete area of the surface, or it can sit partially on the top of the buckles and partially on the air trapped in between the wrinkles creating a composite interface. For convenience, we shall refer to the former as 'Conformal wetting' and denote it by Si ,CNF and the latter as 'Composite wetting' denoted by Si ,CMP . The details of the experiment are provided in the experiment section and the results are

shown in Fig. 6.

Figure 6 Fluorescent microscopic images of the substrates after removing the dye added DI water droplet on surfaces of different pre-strain. (a) χ = 20% ; (b) χ = 35% ; (c) χ = 65% . The dark shades, clearly observed in Fig. 6, represent the places not wetted by the dye containing droplet. The percentage of area under the droplet where the liquid makes physical contact with the surface can thus be measured by the fraction of the bright regime. We have found that the percentage of the bright areas are higher than 90 % for χ = 20% , χ = 35% , (shown here) as well as for χ = 50% , but sharply decrease to 44.2 % for χ = 65% . This implies that with increasing

χ of the substrates, the wetting behavior changes from conformal wetting to a composite wetting state. We postulate here that the higher A and shorter λ signifies a smaller radius of curvature of the water droplet on the wrinkles and hence higher Laplace pressure. This higher Laplace pressure in the droplet, between the wrinkles on substrates of higher χ , restricts the 11


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 30

fluid from going into the crests between the two consecutive peaks, and in turn, prohibits a conformal wetting state. Thus, on surfaces with higher χ , a composite interface is observed. Therefore, the wrinkle topographies observed during this study can be divided into two groups based on their initial wetting states, namely as Si ,CNF and Si ,CMP . The wetting state exhibited by a droplet on a given substrate can be found from geometrical analysis of the surface profile as well. A droplet would have a composite interface only when the tangent to the wrinkle profile at the average height of the protrusions is greater than the tangent of the droplet contact angle on a flat surface46. Due to the sinusoidal nature of the wrinkled surface profile, the condition can mathematically be written as

2π A

λ Henceforth, the term

2π A

λ

> tan( −θ e )

(3)

is referred as the wetting parameter (P) because of its important role

in deciding the wetting regime and tan( −θ e ) is called as the critical wetting parameter (Pc). The situation is further explained schematically in Fig. 7.

Figure 7 A schematic of the two wetting states and the geometric conditions. From the experimentally obtained values of A and λ , the necessary conditions for conformal (or composite) wetting are evaluated and compared with the experimentally obtained wetting states (as in Fig. 7) and the results are summarized in Table 1. 12


Page 13 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Table 1 Variation of wetting characteristics of the wrinkled surfaces with pre-strain. Theoretically obtained

χ

( in % )

λ ( µ m)

A ( µ m)

r⊥

Pc

P

state of wetting (from Eq. 3.)

20

47±2

35

42±1.8

3.55±0.25 1.054

8±0.5

0.4746

1.293

1.1968

tan(−113° )

= 2.3558

50

32±1.9

10±0.6

1.657

1.9635

65

30±1.5

12±0.55

1.950

2.5132

80

28±1.25

13±0.68

2.175

2.9172

Experimentally observed state of wetting (from Fig. 6)

Conformal

Conformal

Wetting

Wetting

Conformal

Conformal

Wetting

Wetting

Conformal

Conformal

Wetting

Wetting

Composite

Composite

Wetting

Wetting

Composite

Composite

Wetting

Wetting

Since the value of P is proportional to the ratio of the amplitude and the wavelength of the wrinkle surface, the wetting state changes from a conformal wetting to a composite wetting regime beyond a certain value of χ for which the value of P is higher than

Pc . Also, from Table 1 it can be observed that the experimental observations are in good agreement with the theoretical predictions. To further investigate this effect of anisotropy on droplet spreading, we carried out additional EWOD experiment on these surfaces, to augment and examine the enhanced spreading of the droplet. The additional phenomenon of electric field assisted transition of wetting states of the droplet - from the Cassie-Baxter (composite wetting) to Wenzel (Conformal wetting) was also looked into. However, in the current study not all surfaces initially exhibited a composite wetting regime, thus opening a new facet to the problem; that is the effect of an initial Wenzel state upon electrowetting. 13


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 30

3.4. Enhanced EWOD. To investigate electrowetting on these structured surfaces with unidirectional wrinkles, we have measured the θ and θ⊥ as functions of the applied voltage for different values of χ and the results are presented in Fig. 8.

Figure 8 The variation of contact angle (θ ) with applied voltage (V ) in an EWOD set-up for different χ ( χ = 20 % to 80 % ) . It can be seen from Fig. 8 that the electrowetting curves deviate from the parabolic nature as predicted by the Young-Lipmann equation. A summary of the measured electrowetted contact angles and their dependency on the initial surface topography which is a function of the amount of pre-straining is presented in Table 2. Although in general, the values of θ and θ⊥ decreases steadily with an applied voltage; few distinct trends can be observed from 14


Page 15 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Fig. 8 and are listed herein - i) θ⊥ is always higher than θ , and the total reduction in θ⊥ is higher than θ over the experimental voltage ranges for all the substrates; ii) the threshold voltage (the voltage at which the droplet shows the first measureable decrease in contact angle signifying spreading) for θ⊥ decreases with increase in χ , whereas the threshold voltage for θ is almost constant. As mentioned previously, on the wrinkled surface, the droplet encounters physical barriers (in the form of crests of the wrinkles) to spreading in the direction perpendicular to the wrinkles, while in the parallel directions the wrinkles aid the spreading due to capillary action. Thus, threshold voltage varies in the two orthogonal directions of the droplet; iii)the total reduction in θ⊥ increases with increase in χ for Si ,CNF (72° - 80°), and then decreases to a lower value (~72°) for Si ,CMP ; iv) on the Si ,CMP , ( χ = 65% and

80 % ) , θ⊥ decreases at a

faster rate for lower voltages and beyond a certain voltage (~300V-400V) decreases at a slower rate. It needs to be mentioned that the maximum change in contact angles achieved during electrowetting in this study (i.e. ~80°) is noticeably higher than those reported for electrowetting experiments on micro-structured surfaces44. The reasons for the above-mentioned observations are discussed below. The continuance of the trend ( θ ⊥ > θ ) on all surfaces for the entire range of applied voltages is a result of the unidirectional nature of the structures. The roughness factors in the two perpendicular directions are very different (values provided in Section 3.1) causing this anisotropy in the wetting behavior of the droplet. However, upon further observation it can be seen that the threshold voltage for the perpendicular direction decreases with increase in the wrinkle compactness on Si ,CNF . With increase in the pre-strain (till χ = 50% ), the capillarity increases due to the formation of narrower wrinkles, and the drop spreads more in the parallel direction due to the capillary action provided by the wrinkles as compared to the perpendicular direction. Additionally, with increase in the applied potential the droplet spreads more in the parallel direction due to the combined effect of electrowetting and capillarity. This reduces θ ⊥ as well, since the droplet gets extended in one of the planar directions (parallel to the wrinkles in this case). Therefore, a better metric to examine the phenomenon is the droplet footprint radius

15


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 30

and the eccentricity, which has been measured in the subsequent section and the results are found to be consistent with the physics of the process. The droplet on the initially conformal surface, Si ,CNF elongates marginally in the direction of wrinkles even in the absence of electric field (Fig. 5). Thus, for Si ,CNF the droplet experiences capillary action that causes it to preferentially spread in the direction of the wrinkles. This capillary action assisted by electrowetting, results in higher reduction in the contact angle during the electrowetting experiments. It must also be noted here that the surfaces are predominantly hydrophobic and thus without any electric potential, there will be no spreading of the droplet on these surfaces due to capillary action alone. The relative significance of electrical forces and capillary wicking for the initially conformal surfaces can be obtained following the approach used in47 and is presented in the following table for higher applied voltages where the effects of the electrical forces start to become significant. It is clear that for a constant stretching, the effects of electrical forces start to become significant with increase in the applied potential (the ratio increases from 0.155 to 0.532 for an increase in the applied potential from 300 V to 600 V for χ = 50%). The increase in the capillary forces with increase in stretching (signifying narrower wrinkles) at a fixed voltage is also apparent. Table 2 Relative effects of the capillary and electrowetting forces on substrates of varying stretching showing initial conformal states. F cap / F electric

Voltage (V)

χ = 20%

χ = 35%

χ = 50%

300

0.013

0.069

0.155

400

0.031

0.167

0.375

500

0.039

0.212

0.476

600

0.044

0.237

0.532

16


Page 17 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

The justification for the third observation can be inferred from the fact that for Si ,CNF , the droplet penetrates the micro-grooves that provides capillary wicking. The capillary wicking effect increases with increase in χ , till the initial conformal wetting regime changes into a composite interface. The sudden decrease in the value of total reduction in θ⊥ on Si ,CMP is due to the decrease in the capillary wicking. The fourth observation can be explained by the transition in the wetting state from the composite to the conformal. Considering the variation in κ (the fraction of solid-liquid contact area to the total projected area), it has recently been shown by energy minimization approach that the reduction in the contact angle is faster when the droplet is in the composite state ( κ < 1 ). Once the transition into the conformal wetting state ( κ > 1 ) takes place, the rate of reduction of the contact angle decreases48.The change in contact angle per unit change in the applied voltage are calculated and presented in Fig. 9. The figure consists of the results of two distinct cases- the left one depicts the behavior when the droplet on the substrate is initially in the conformal state, while the right one is for initial composite state. The salient features of the figure are as follows.

Figure 9 Rate of change of contact angles with voltage for wrinkled surfaces of different prestrain. The left graph depicts the trend on surface with initial conformal wetting state ( χ = 35% ) and the right one shows the trend on surface exhibiting initial composite state ( χ = 65% ). The lines are for the viewer’s eyes only. The behavior of the change in the contact angle in two orthogonal directions is different for initial conformal and composite states. In the initially conformal state, the rate of decrease in contact angle is substantially different in the two directions up to about 300V. The 17


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 30

behavior can be attributed to the additional structural obstacles to spreading due to the crests of the wrinkles encountered by the droplets in conformal states. Once this barrier is crossed (at higher voltages), the rate of contact angle change is more isotropic. On the initially composite states, the difference in the rates of contact angle change in the two directions is noticeably smaller. On further observation of the graphs for initially composite states, Si ,CMP (for χ = 65% ), a sharp fall in the rate of the contact angle change in both the orthogonal directions can be noticed at around 300V which was absent in the initially conformal states. It has also been mentioned in a recent study48 that for the composite state the droplet undergoes faster change in the contact angle whereas after transition into the conformal state the rate of change of contact angle decreases. Therefore, it can be inferred that the droplet on the Si ,CMP is undergoing a transition of wetting states from composite to conformal as the applied electric field is progressively increased. From the third observation, we found that the droplet spreads more on Si ,CNF than on Si ,CMP . This underscores the fact that conformal state is more amenable to droplet spreading due to the wicking action of the capillaries formed by the wrinkles. However, the fourth observation reveals that for the Si ,CMP , spreading is more pronounced while the droplet is in the composite state than in the conformal state. During the unhindered spreading of the droplet (for the composite state of wetting), the droplet spreads easily in all directions with no capillary effect till the transition of wetting states takes place. Due to the already spread (isotropic) condition of the droplet, the effect of capillary action remains minimal. (see the videos in the Supporting Information for details). To corroborate the transition in the wetting regimes and the initial unhindered spreading of the droplet, the measurements of the diameters are needed and the results of these measurements are reported in the next subsection.

18


Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Table 3 The changes in the contact angle during electrowetting as a function of the structured surface.

χ

θ

∥

⊥

20%

113±3

122±2

35%

Initial

Minimum Electro-

Maximum change in

State of

wetted Contact

Electro wetted

Wetting

Angle

Contact Angle

∥

⊥

∥

⊥

Conformal

46±1.8

50.1±0.8

67±3.49

72±2.15

113±4

131±1.56 Conformal

45±1.08

52±0.4

68±4.14

79±1.61

50%

113±3.64

143±2.14 Conformal

44±1.6

63±0.56

69±3.97

80±2.21

65%

116±3.04

144±2.55 Composite

63±3

74±0.8

53±4.27

70±2.67

80%

119±3.4

150±2.22 Composite

64±0.8

75±0.89

55±3.49

75±2.39

3.5. Tunability of droplet anisotropy: To explore the anisotropy during electrowetting, we have measured the diameters of the droplet footprint along the two perpendicular directions with respect to the wrinkles' direction ( D and D⊥ ) not only during the increment in voltage but also with the gradual reduction of the same, and the results are presented in Fig. 9.

19


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 30

Figure 10 Comparison of the diameters in parallel and perpendicular directions upon application of voltages, on surfaces of varying wrinkle parameters. It is clear from Fig. 10 that irrespective of the χ values, D starts increasing up to a voltage of 600V. As the voltage is progressively reduced, D starts to decrease with minimal hysteresis. The nearly reversible electrowetting in the parallel direction is due to the capillary suction provided by the wrinkles and the associated reduction in the obstruction to the movement of the contact line along the wrinkles. On the other hand, on substrates of lower χ (signifying the presence of Si ,CNF ), D⊥ initially decreases for lower voltages, due to the rapid elongation of the droplet in the parallel direction, and then beyond a threshold voltage, it starts increasing till the highest voltage (600V as used in this study) is reached. The additional stretching of the droplet in the parallel direction results in a small yet measureable reduction in D⊥ as the volume of the droplet remains constant during the spreading and results in a decrease of the contact angles. 20


Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Thus, it is postulated that the initial reduction in θ⊥ is due to the spreading of the droplet in the parallel direction to a greater extent and not due to the spreading in the perpendicular direction. It can be observed from Fig. 10 that for Si ,CNF , the difference between D and

D⊥ , increases with increase in the value of χ . This is due to the increase in compactness of the wrinkles with increase in χ , which engenders higher capillary wicking and thus enhanced spreading during electrowetting. This increase in the spreading with increasing χ takes place only for the wetting state Si ,CNF . The electrowetting of the droplet on Si ,CMP depicts a slightly different trend. The simultaneous increases in D⊥ along with D for lower voltages depict the isotropic nature of spreading on these surfaces. The contact angle changes with increasing voltages are such that the trend lines representing θ and θ⊥ remain approximately parallel in Fig. 8 for ( χ = 80 % ) . The droplet spreads in the perpendicular direction more easily on Si ,CMP (as compared to Si ,CNF ) due to the presence of the air-liquid interface, resulting in lesser obstructions provided by the wrinkles. Also, the capillary wicking is missing in the initial stages of electrowetting which is the only factor responsible for elongation of the droplet in the parallel direction. Thus, the total reduction in θ ⊥ increases with the increase in χ , for Si ,CNF , and after the transformation of the wetting state to Si ,CMP it starts to decrease. It can be observed from Fig. 10 that on Si ,CMP (65% and 80%) , D⊥ monotonously increases (instead of decreasing) at lower voltages, till the highest applied voltage is reached. During the reduction of the voltage, D⊥ remains constant, and only after the voltage has reduced to a much lower value, the droplet starts retracting in the perpendicular direction. The droplets are not able to revert to their initial shape due to the obstructions posed by the wrinkles in the perpendicular direction requiring additional forces needed to overcome the crests of the wrinkles while retracting. This phenomenon takes place for all surfaces irrespective of their initial wetting state, signifying a transition of wetting state on the Si ,CMP surfaces. Also, it can be observed from Fig. 10 that D and D⊥ both increases simultaneously for ( χ = 65% and χ = 80% ) at lower applied voltages, depicting an isotropic spreading. Only after the transition of wetting state from 21


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 30

composite to conformal wetting, the capillary wicking comes into action. The effect of electric field is more pronounced in the composite state in comparison to the conformal case as in the later the capillary effects dominate. These observations provide additional support to the fourth observation of the last subsection and the subsequent postulate. This anisotropic retraction, after a complete electrowetting cycle, strongly depends upon the surface topography. A clearer depiction of this phenomenon can be obtained by comparing the values of ε for two different surfaces ( χ = 35% , and χ = 65% ). The images of the drops from the top for intermediate voltages are given in Fig. 11, and the values of eccentricity are plotted in Fig. 12.

Figure 11 Comparison of the anisotropic wetting of the droplet on lower pre-strained substrates (top) to that on the higher pre-strained substrates (bottom) for a cyclic potential difference. The anisotropy during spreading is more for Si ,CNF than Si ,CMP , whereas, after retraction (post electrowetting cycle), the droplet on Si ,CMP becomes more anisotropic compared to Si ,CNF but in getting more elongated in perpendicular direction.

22


Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure 12 Eccentricity variation with imposed voltage for substrates of varying pre-strains ( χ = 20% , χ = 35% , χ = 50% , χ = 65% and χ = 80% ). From Fig. 12, it can be observed that ε initially increases for the substrates, reaches a maximum and starts to decrease with further increase in the voltage, as the droplet starts spreading in the perpendicular direction as well. Initially, the droplet requires more energy to cross the wrinkles as compared to spread along the wrinkles. This threshold voltage required by the droplet to cross the wrinkles, decreases with change in the initial wetting state from conformal to composite (300 V for χ = 35% to 100V for χ = 65% , as can be seen in Fig. 10). This is also manifested in the form of reduced anisotropy on Si ,CMP as compared to Si ,CNF during the droplet spreading process as can be seen in Fig. 11. The values of ε keep on decreasing with reduction in the applied potential. This is due to a higher energy barrier faced by the droplet in the perpendicular direction engendered by the wrinkles during retraction and a reversible retraction in the parallel direction.

The

irreversible

retraction

of

a

droplet

in

the

perpendicular

direction

on Si ,CMP ,represents the transition from a composite state to a conformal state since the wrinkles can hinder the retraction of the droplet in conformal state only. For Si ,CNF , the anisotropy increases with increase in χ due to the higher capillary wicking provided by the increasing 23


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

number of wrinkles (per unit length of the surface) with increasing χ . This increase in anisotropy is however a strong function of the initial wetting state. The anisotropy decreases drastically when χ increases beyond a critical value at which the initial interfacial wetting state changes to a composite one. The lower value of ε for Si ,CMP is yet another indicator of the isotropic spreading on these surfaces. Since the final diameter in the perpendicular direction on the Si ,CMP is higher compared to Si ,CNF , it can be observed that the droplet, unlike its initial

orientation, remains more elongated in the perpendicular direction after one electrowetting cycle. Thus, a reversal in the anisotropy ( ε > 1 to ε < 1 ) of a droplet has been observed during electrowetting on the Si ,CMP , and is clear from the images provided in Fig. 10 ( χ = 65% ). Thus, a judicious choice of the wrinkle parameters and the applied electric potential may result in a desired anisotropy on a wrinkled substrate.

4. Conclusion A novel method to fabricate a stamp-less and mask-less fabrication technique to create structured surfaces is presented herein using unidirectional stretching of a PDMS film followed by deposition of nichrome through sputtering. The size and the shape of the wrinkles thus obtained can be controlled by varying the pre-strain applied to the elastomeric substrate. The directionality of the wrinkles gives rise to anisotropic wetting on the substrate. It has been shown that the anisotropy decreases with the pre-strain beyond a certain value due to the change in the initial wetting state from a conformal wetting to a composite wetting state, thereby reducing the capillary effect provided by the micro-grooves along the wrinkles. Electrowetting studies carried out on these surfaces results in changes in the contact angles which are appreciably higher than the previous electrowetting results on micro-structured surfaces, and are strongly dependent on the initial wetting state. The initial composite wetting states are suitable for faster spreading at lower voltages, whereas the conformal states yield higher anisotropic spreading and better directional maneuverability of droplets. The study establishes that by tuning the pre-strain, it is possible to significantly alter droplet spreading during electrowetting and indicates the potential of fast and enhanced droplet manipulation on structured surfaces in microfluidic applications.

24


Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Acknowledgements The authors gratefully acknowledge the financial support provided by Indian Space Research Organization (ISRO) through the Kalpana Chawla Space Technology Cell, Indian Institute of Technology Kharagpur, India [Sanction Letter No: IIT/KCSTC/Chair./NEW/P/17-18/01, Dt. 1705-2017].

Supporting Information Videos 1and 2: Video of a droplet undergoing electrowetting at 300 V on two different surfaces (Video 1: for electrowetting on χ = 50% substrate; Video 2: for electrowetting on χ = 80% substrate) depicting the different modes of droplet spreading. Videos are played at 1.75 times the actual speed.

References (1)

Yan, Y. Y.; Gao, N.; Barthlott, W. Mimicking Natural Superhydrophobic Surfaces and Grasping the Wetting Process: A Review on Recent Progress in Preparing Superhydrophobic Surfaces. Advances in Colloid and Interface Science2011, 169 (2), 80–105.

(2)

Sanjay, S. L.; Annaso, B. G.; Chavan, S. M.; Rajiv, S. V. Recent Progress in Preparation of Superhydrophobic Surfaces: A Review. Journal of Surface Engineered Materials and Advanced Technology2012, 2 (2), 76–94.

(3)

Sun, Y.; Qiao, R. Facile Tuning of Superhydrophobic States with Ag Nanoplates. Nano Res2008, 1, 292–302.

(4)

Mugele, F.; Baret, J.-C. Electrowetting: From Basics to Applications. Journal of Physics: Condensed Matter2005, 17 (28), R705–R774.

(5)

Park, S.-Y.; Kalim, S.; Callahan, C.; Teitell, M. a; Chiou, E. P. Y. A Light-Induced Dielectrophoretic Droplet Manipulation Platform. Lab on a chip2009, 9 (22), 3228–3235.

(6)

García, A. A.; Egatz-Gómez, A.; Lindsay, S. A.; Domínguez-García, P.; Melle, S.; Marquez, M.; 25


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

Rubio, M. A.; Picraux, S. T.; Yang, D.; Aella, P.; et al. Magnetic Movement of Biological Fluid Droplets. Journal of Magnetism and Magnetic Materials2007, 311 (1 SPEC. ISS.), 238–243. (7)

Valentino, J. P.; Darhuber, A. A.; Troian, S. M.; Wagner, S. Thermocapillary Actuation of Liquids Using Patterned Microheater Arrays. In TRANSDUCERS 2003 - 12th International Conference on Solid-State Sensors, Actuators and Microsystems, Digest of Technical Papers; 2003; Vol. 1, 667– 669.

(8)

Wixforth, A.; Strobl, C.; Gauer, C.; Toegl, A.; Scriba, J.; Guttenberg, Z. V. Acoustic Manipulation of Small Droplets. Analytical and Bioanalytical Chemistry2004, 379 (7–8), 982–991.

(9)

Peng, C.; Zhang, Z.; Kim, C.-J. C. J.; Ju, Y. S. EWOD (Electrowetting on Dielectric) Digital Microfluidics Powered by Finger Actuation. Lab on a chip2014, 14 (6), 1117–1122.

(10)

Srinivasan, V.; Pamula, V. K.; Fair, R. B. An Integrated Digital Microfluidic Lab-on-a-Chip for Clinical Diagnostics on Human Physiological Fluids. Lab on a chip2004, 4 (4), 310–315.

(11)

Yasuda, T.; Imamura, K.; Hirase, K. Droplet Transportation Using EWOD-Induced Wettability Gradient. In TRANSDUCERS 2009 - 15th International Conference on Solid-State Sensors, Actuators and Microsystems; 2009; 413–416.

(12)

Chang, B.; Shah, A.; Routa, I.; Lipsanen, H.; Zhou, Q. Surface-Tension Driven Self-Assembly of Microchips on Hydrophobic Receptor Sites with Water Using Forced Wetting. Applied Physics Letters2012, 101 (11), 114105/1-5.

(13)

Banpurkar, A. G.; Nichols, K. P.; Mugele, F. Electrowetting-Based Microdrop Tensiometer. Langmuir2008, 24 (19), 10549–10551.

(14)

Kim, D. Y.; Steckl, A. J. Electrowetting on Paper for Electronic Paper Display. ACS Applied Materials and Interfaces2010, 2 (11), 3318–3323.

(15)

Kuo, J. S.; Spicar-Mihalic, P.; Rodriguez, I.; Chiu, D. T. Electrowetting-Induced Droplet Movement in an Immiscible Medium. Langmuir2003, 19 (2), 250–255.

(16)

Ren, H.; Fair, R. B.; Pollack, M. G. Automated on-Chip Droplet Dispensing with Volume Control by Electro-Wetting Actuation and Capacitance Metering. Sensors and Actuators, B: Chemical2004, 98 (2–3), 319–327.

(17)

Fowler, J.; Moon, H. M. H.; Kim, C.-J. K. C.-J. Enhancement of Mixing by Droplet-Based 26


Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Microfluidics. Technical Digest. MEMS 2002 IEEE International Conference. Fifteenth IEEE International Conference on Micro Electro Mechanical Systems (Cat. No.02CH37266)2002, 97– 100. (18)

Cho, S. K.; Moon, H.; Fowler, J.; Kim, C.-J. Splitting a Liquid Droplet for Electrowetting-Based Microfluidics. In ASME International Mechanical Engineering Congress and Exposition, Proceedings; 2001; Vol. 2, 2781–2787.

(19)

Nguyen, N.-T.; Shaegh, S. A. M.; Kashaninejad, N.; Phan, D.-T. Design, Fabrication and Characterization of Drug Delivery Systems Based on Lab-on-a-Chip Technology. Advanced drug delivery reviews2013, 65 (11–12), 1403–1419.

(20)

Berge, B.; Peseux, J. Variable Focal Lens Controlled by an External Voltage: An Application of Electrowetting. The European Physical Journal E2000, 3 (3), 159–163.

(21)

Hayes, R. A.; Feenstra, B. Video-Speed Electronic Paper Based on Electrowetting. Nature2003, 425 (6956), 383–385.

(22)

Verplanck, N.; Galopin, E.; Camart, J. C.; Thomy, V.; Coffinier, Y.; Boukherroub, R. Reversible Electrowetting on Superhydrophobic Silicon Nanowires. Nano Letters2007, 7 (3), 813–817.

(23)

Dhindsa, M. S.; Smith, N. R.; Heikenfeld, J.; Rack, P. D.; Fowlkes, J. D.; Doktycz, M. J.; Melechko, a V; Simpson, M. L. Electrowetting on Arrayed Carbon Nanofibers. Nanotechnology, 2006. IEEE-NANO 2006. Sixth IEEE Conference on2006, 1, 207–210.

(24)

Bahadur, V.; Garimella, S. V. Electrowetting-Based Control of Droplet Transition and Morphology on Artificially Microstructured Surfaces. Langmuir2008, 24 (15), 8338–8345.

(25)

Han, Z.; Tay, B.; Tan, C.; Shakerzadeh, M.; Ostrikov, K. Electrowetting Control of Cassie-toWenzel Transitions in Superhydrophobic Carbon Nanotube-Based Nanocomposites. ACS Nano2009, 3 (10), 3031–3036.

(26)

Barberoglou, M.; Zorba, V.; Pagozidis, A.; Fotakis, C.; Stratakis, E. Electrowetting Properties of Micro/nanostructured Black Silicon. Langmuir2010, 26 (15), 13007–13014.

(27)

Bruning, J. H. Optical Lithography: 40 Years and Holding. Proceedings of SPIE2007, 6520, 652004–652013.

(28)

Chang, S.; Yoon, J. Arrays Fabricated by a 3D Diffuser Lithography. Optics Express2004, 12(25), 27


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

6366–6371. (29)

Qin, D.; Xia, Y.; Whitesides, G. M. Soft Lithography for Micro- and Nanoscale Patterning. Nature protocols2010, 5 (3), 491–502.

(30)

Kovacs, G. T. A.; Maluf, N. I.; Petersen, K. E. Bulk Micromachining of Silicon. Proceedings of the IEEE1998, 86 (8), 1536–1551.

(31)

Marty, F.; Rousseau, L.; Saadany, B.; Mercier, B.; Français, O.; Mita, Y.; Bourouina, T. Advanced Etching of Silicon Based on Deep Reactive Ion Etching for Silicon High Aspect Ratio Microstructures and Three-Dimensional Micro- and Nanostructures. Microelectronics Journal. 2005, 673–677.

(32)

Bowden, N.; Brittain, S.; Evans, A. G.; Hutchinson, J. W.; Whitesides, G. M. Spontaneous Formation of Ordered Structures in Thin Films of Metals Supported on an Elastomeric Polymer. Nature1998, 393, 146–149.

(33)

Langley, K. R.; Sharp, J. S. Microtextured Surfaces with Gradient Wetting Properties. Langmuir2010, 26 (23), 18349–18356.

(34)

Roy, P. K.; Pant, R.; Nagarajan, A. K.; Khare, K. Mechanically Tunable Slippery Behavior on Soft Poly(dimethylsiloxane)-Based Anisotropic Wrinkles Infused with Lubricating Fluid. Langmuir2016, 32 (23), 5738–5743.

(35)

Goel, P.; Kumar, S.; Sarkar, J.; Singh, J. P. Mechanical Strain Induced Tunable Anisotropic Wetting on Buckled PDMS Silver Nanorods Arrays. ACS Applied Materials and Interfaces2015, 7 (16), 8419–8426.

(36)

Hiltl, S.; Bo, A. Wetting Phenomena on ( Gradient ) Wrinkle Substrates. Langmuir2016, 32 (35), 8882–8888.

(37)

Maji, D.; Das, D.; Wala, J.; Das, S. Buckling Assisted and Lithographically Micropatterned Fully Flexible Sensors for Conformal Integration Applications. Scientific reports2015, 5, 17776–17792.

(38)

Zhou, Q.; Kühn, P. T.; Huisman, T.; Nieboer, E.; Zwol, C. Van. Directional Nanotopographic Gradients : A High-Throughput Screening Platform for Cell Contact Guidance. Scientific reports2015, 5, 1–12.

(39)

Chung, J. Y.; Youngblood, P.; Stafford, C. M. Anisotropic Wetting on Tunable Micro-Wrinkled 28


Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Surfaces. Soft Matter2007, 3 (9), 1163–1169. (40)

Li, W.; Fang, G.; Li, Y.; Qiao, G. Anisotropic Wetting Behavior Arising from Superhydrophobic Surfaces : Parallel Grooved. Journal of Physical Chemistry B2008, 112 (24), 7234–7243.

(41)

Demirel, M. C. Unidirectional Wetting Properties. Nature Materials2010, 9 (12), 1023–1028.

(42)

Herbertson, D. L.; Evans, C. R.; Shirtcliffe, N. J.; McHale, G.; Newton, M. I. Electrowetting on Superhydrophobic SU-8 Patterned Surfaces. Sensors and Actuators, A: Physical2006, 130–131 (SPEC. ISS.), 189–193.

(43)

Lingbo, Z.; Jianwen, X.; Yonghao, X.; Yangyang, S.; Hess, D. W.; Ching-Ping, W. Electrowetting of Aligned Carbon Nanotube Films. Journal of Physical Chemistry B2006, 110 (32), 15945– 15950.

(44)

Im, M.; Kim, D. H.; Lee, J. H.; Yoon, J. B.; Choi, Y. K. Electrowetting on a Polymer Microlens Array. Langmuir2010, 26 (14), 12443–12447.

(45)

Vrancken, R. J.; Kusumaatmaja, H.; Hermans, K.; Prenen, A. M.; Pierre-Louis, O.; Bastiaansen, C. W. M.; Broer, D. J. Fully Reversible Transition from Wenzel to Cassie-Baxter States on Corrugated Superhydrophobic Surfaces. Langmuir2010, 26 (5), 3335–3341.

(46)

Zhang, B. J.; Park, J.; Kim, K. J.; Nosonovsky, M.; Bhushan, B. Roughness-Induced Superhydrophobicity : A Way to Design Non-Adhesive Surfaces. Journal of Physics: Condensed Matter2008, 20 (22), 225009–225040.

(47)

Kim, J.; Moon, M.-W.; Kim, H.-Y. Dynamics of Hemiwicking. Journal of Fluid Mechanics2016, 800, 57–71.

(48)

Wang, Z.; Zhao, Y.; Wang, Z.; Zhao, Y. Wetting and Electrowetting on Corrugated Substrates. Phys. Fluids2017, 29 (6), 67101–67110.

29


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 30

For Table of Contents Use Only

30


Anisotropic Electrowetting on Wrinkled Surfaces: Enhanced Wetting

Recommend Documents